This invention relates to a wear-resistant titanium alloy and articles made thereof. In particular, it relates to a titanium alloy for use in articles such as automobile valve parts (such as engine valves, springs, and retainers), and steam turbine blades which exhibit improved resistance to sliding abrasive wear and erosion when subject to collision with high speed droplets. The alloy is light in weight, is easily deformable by hot rolling, and is weldable to other articles made of titanium or titanium alloys.
Recently, production techniques for titanium alloys have improved to the point that they are now manufactured on an industrial scale. As a result, titanium alloys are being applied to an increasing variety of articles, which take advantage of the high specific strength, good corrosion resistance, and good thermal resistance of these alloys. On the other hand, titanium alloys are also known to have low resistance to wear in a dry state, so it is quite difficult to use titanium alloys for portions of mechanical parts which are subject to sliding contact with other parts. Therefore, it is necessary to apply a wear-resistant treatment to articles such as automobile parts (e.g. engine valves) which must have good resistance to wear.
One commercially-available wear-resistant material is "Stellite" (trade name), which is known for its excellent resistance to wear. Stellite has been widely used as a hard-facing or bonding material for application to surfaces of machine parts which are subjected to abrasive wear.
There have also been attempts to apply Stellite to the surface of titanium alloys so as to improve the resistance to wear. However, although it is possible to effect hard-facing and bonding of Stellite to ferrous materials, it is impossible to do so with respect to titanium alloys. It is impractical to use Stellite so far as titanium alloys are concerned.
Therefore, "nitriding", "plating with metals such as Ni and Cr", "vapor deposition (i.e., PVD, and CVD)", or "carburizing" have been employed to form a wear-resistant film on the surface of machine parts made from titanium alloys.
A type of hardening treatment by hard-facing has been proposed so as to improve the wear resistance of titanium alloy articles. Japanese Published Unexamined Patent Application No. 61-231151 discloses a method in which hardening materials such as metal oxide (e.g., TiO.sub.2), metal carbide, metal nitride or oxygen are placed onto the surface of articles made of titanium alloy, and then the hardening materials are irradiated with a high energy beam to fuse the hardening materials and form a uniform surface layer.
Japanese Published Unexamined Patent Application No. 62-56561 proposes irradiating the surface of a titanium alloy article with a high energy beam to fuse the surface, after which hardening materials such as TiN and solid-solution hardening materials such as oxygen are injected into the resulting molten pool.
However, the conventional nitriding and carburizing methods are accompanied by the formation of thermal strains, since the articles to be treated are exposed to high temperatures. It has also been pointed out that the hard coatings which are obtained by metal plating or vapor deposition are easily peeled off. Hardening by hard-facing can effect hardening of the overlays, but matching of the hardness of the hard-facing with that of the mother material being treated (e.g. ferrous materials) is not satisfactory, sometimes resulting in wearing not only of the overlays but also of the mother material. In addition, there are many cracks in the hard-facing layer and segregation of hardening materials is inevitable.
On the other hand, in wet corrosive conditions the wear resistance of titanium alloys is not as critical as in dry corrosive conditions under mild conditions. However, in the case of steam turbine blades, titanium alloys cannot exhibit a satisfactory level of resistance because of severe erosion caused by high speed droplets. For this purpose .beta.-type titanium alloys such as Ti-15Mo-5Zr alloys and Ti-15Mo-5Zr-3Al alloys are used after aging as an erosion-shielding material for steam turbine blades made of a Ti-6Al-4V alloy. Aged .beta.-type titanium alloys are relatively hard compared with the other titanium alloys.
However, such aged .beta.-type titanium alloys do not have the same level of resistance against the droplet erosion as Stellite, which is successfully applied to turbine blades made of ferrous materials.